The invention is generally related to laying cable from a vessel and more particularly to laying cable to or from shore.
Traditionally, the standard methodology for laying a subsea cable to or from shore would be by either of the following two methods.
In the first method, a cable lay vessel (CLV) loaded with the full length of cable sets up at a distance from the shore ready to pay out the cable end. A winch wire from shore is connected to the cable end and begins pulling the cable towards the shore. Floatation buoys are attached at intervals as the cable is progressively paid out towards the shore. This method has significant limitations (especially with relatively large diameter, heavy cables), including effects of weather, currents, and lack of water depth for the buoyancy to be effective. This method also requires the mother ship to come relatively close in to the shore (usually, about one kilometer, maximum distance from shore) and imposes significant limitations of the selection of a suitable cable landfall site with sufficient water depth for the CLV.
The second method provides an alternative, which is to install a conduit from shore under the seabed using directional drilling with the conduit exiting the seabed at a suitable distance and water depth offshore. This may be appropriate where the seabed topography of the shore approach is not conducive to the integrity of the cable and long term service life or possibly for other restrictions such as environmental considerations. The conduit exit point is at a location where the CLV can set up to receive the end of a pre-installed winch wire to be connected to the cable end. The cable is then paid out from the CLV and pulled through the conduit by the shore winch. As with the first method above, this restricts selection of a suitable landfall site to where the CLV can approach to within one to two kilometers from shore (dictated by the maximum distance it would be feasible to pull a cable through the conduit).
The problem of proximity to shore for the CLV is overcome by transferring a portion of the cable from the CLV to a shallow water barge which would then lay cable towards the shore. However, when the barge can proceed no further, the cable must then be transferred to the final tie-in point onshore. Traditionally, this would be performed by floating in a large loop (bight) until the free end is available and can be connected to a shore winch wire. The floating loop is then pulled in until it can be positioned along the planned cable route. This method shares the same limitations as in the first method above, such as weather and cross currents due to the cable being in a large loop.